new number confirming a worry in american families. Tonight, one in 50 schoolchildren in america has a form of autism. The government survey is much higher than the previous estimate, 1 in 88. So what is happening? And also tonight, another small part of the puzzle. Does it have anything to do with parents and grandfathers? Abc's chief medical editor, dr. Richard besser, on the clues. Reporter: And now another clue into a possible cause of autism -- grandfather's age. Researchers in sweden have found that the older a grandfather was when he had children, the more likely those children would be to have children with autism. For fathers the risk went up 1.8 times. For mothers, the risk went up 1.7 times. It's already known that one piece of the puzzle of autism is the age of the father. Starting at age 30. Take ray siegel. Let's figure out if theres a genetic factor. Both his sons have autism. Struggling with reading to social interaction. Dads pass on 25 new mutations at age 20, increasing to 65 mutations at age 40 and so on. The more mutations, the greater the risk of autism. But what does this have to do with grandfathers? So we could find for example, that in a grandfather, there was a change in the grandfather's dna that then was passed on to the father, and then finally contributed to risk for autism in that father's child. Keep in mind, researchers say grandfather's age, a clue to the cause of autism, could only account for at about 3% of the risk. One in 50 schoolchildren in america, is this a big jump? It is a jump. A large part of it is probably better recognition, which is good, because early treatment is a better outcome. They think some of this is more children with autism and that's

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